Known vertical form/fill/seal machines are disclosed in U.S. Pat. Nos. 3,894,381 to Christine et al. and 4,246,062 to Christine. In general, these machines take a continuous web of thermoplastic material; form it into a continuous sleeve by folding the web and welding its free edge portions together; fill the sleeve with a fluid material as the bottom is sealed shut; and seal the sleeve closed after filling is complete. The machines make and fill the flexible pouches in a continuous manner, so that the bottom seal for the unfilled pouch is usually made simultaneously with the top seal for the filled pouch. These seals are typically made by heat welding the thermoplastic web to itself, however, other sealing methods can be used.
The fluid filled flexible packages produced by the machines must have strong seals to resist bursting at and/or leakage from the seals which may result during handling and/or transport. The "drop test" measures the strength of these packages For example, the drop test for an intravenous (IV) solution pouch, which is suspended over a patient at a height of 5 to 6 feet above the floor, consists of repetitively dropping the filled pouch from a height of 5 to 6 feet. If the pouch withstands the drop, i.e. it does not burst or leak, it passes the test, otherwise it fails.
Bursting at and/or leakage from the seals is a problem with these types of packages. Particularly, the horizontal seals (i.e. top and bottom seals) are most susceptible to this problem. The problem arises, in part, due to dynamic forces on the pouch at the horizontal seal when it is made.
The principal dynamic force arises during fluid filling at the bottom horizontal seal. Before filling, the folded and side sealed web (or sleeve) is flat or two dimensional and is straight between the folded edge and the side fin (i.e. it has a square bottom). However, when fluid is filled into the sleeve, the fluid, seeking its own level, spreads the sleeve to a three dimensional shape by forcing outwardly the sides of the sleeve which in turn causes the bottom to assume a concave shape. This dynamic force exerts an outwardly directed force or spreading force on the bottom horizontal seal. The spreading force weakens the bottom horizontal seal.
Another dynamic force arises during certain special packaging operations. These special packaging operations include the packaging in an inert atmosphere, a nitrogen or other gas atmosphere, a sterile atmosphere and the like. During these operations, a chamber encloses a portion of the machine and the chamber is flooded with the desired atmosphere. The pressure of the flooding atmosphere is greater than the atmospheric pressure. The flooding atmosphere inflates the flat sleeve and deforms it as described above.
The seals, which are often formed by heat sealing, do not immediately solidify to a finished seal. Instead, a finite amount of time is required to heat the web to a welding temperature; hold the webs in bonding contact; release the hot bonded webs; and then allow the sealed web to cool to a finished seal. During cooling, the seal is particularly susceptible to the dynamic forces. Since the seal has not cooled to a finished seal, it has not obtained its greatest strength and the spreading forces pull the web apart, thereby weakening the seal. Additionally, fluid may enter the seal and further weaken it.
In one method, the horizontal seals have been formed by a hot wire which not only seals the web together but cuts one pouch from the next. The hot wire method forms seals with very small surface areas that do not have the strength of seals with greater surface areas. In another method, the horizontal seals have been formed by a pair of rectangular hot jaws with mating surfaces, which are knurled or have discontinuous patterns embossed thereon. These rectangular hot jaws are used to defeat the effects of the dynamic forces, however, they are not always effective.
A different factor affecting the bursting and leaking problem is pleating of the web at the seals. This pleating is due to unequal lengths of opposing webs being welded together. The pleat is an unwanted fold in the web which normally cannot be sealed shut. Thus, the pleat allows fluid to escape. Additionally, since this pleat represents at least two additional layers of material that must be welded together, the seal is weak and allows the pouch to leak.
In one method to overcome the pleating problem, a pair of spring fingers located within the sleeve and which hang down from the filling nozzle and which engage the folded edge and side seal of the sleeve ensure that equal amounts of web are welded together. However, the spring finger that engages the side seal causes weakening of that seal because the side seal is still in a softened state when engaged by the spring finger. Another apparatus used to overcome the pleating problem utilizes a single spring finger that engages the folded edge and a clamp that engages the side sealed edge. The clamp consists of two endless chains; one chain carries a plurality of pins and the other carries a plurality of sockets that mate with the pins. See U.S. patent application Ser. No. 389,098, filed June 16, 1982, now abandoned. The pins and sockets engage the selvage (i.e. that portion of the side sealed edge beyond the weld). The clamp on the sealed side of the sleeve and the spring finger on the folded side of the sleeve cooperate to ensure that equal lengths of web are welded at the horizontal seam. Moreover, the clamp ensures that the sealed edge of the sleeve is always delivered to the same spot on the sealing jaws. The pins, however, puncture the selvage. In some cases, the consumer deems such a pouch unattractive and it is not usable unless it is placed in some over-package, such as a box.
The foregoing problems and attempted solutions are associated with controlling the web as it passes through the machine.
A three side fin pouch refers to a generally rectangular pouch which includes seals on three sides (i.e. the top, bottom and one side) and a fold at the remaining side.
U.S. Pat. Nos. 3,894,381 and 4,246,062 are incorporated herein by reference.